In recent years, higher performance properties have been required for a non-oriented electrical steel sheet to be used as an iron core material of a rotary machine due to a worldwide increase in achievement of energy saving of an electrical apparatus. Recently in particular, as a motor to be used for an electric vehicle or the like, a demand for a small-sized high-power motor has been high. Such an electric vehicle motor has been designed to make high-speed rotation possible to thereby obtain high torque.
A high-speed rotation motor has also been used for a machine tool and an electrical apparatus such as a vacuum cleaner. The outer shape of a high-speed rotation motor for an electric vehicle is larger than that of a high-speed rotation motor for an electrical apparatus. Further, as a high-speed rotation motor for an electric vehicle, a DC brushless motor has been mainly used. In a DC brushless motor, magnets are embedded in the vicinity of an outer periphery of a rotor. In the above structure, the width of a bridge portion in an outer periphery portion of the rotor (the width between magnets from the most outer periphery of the rotor to a steel sheet) is extremely narrow, which is 1 to 2 mm, depending on a place. Therefore, a high-strength steel sheet has been required for a high-speed rotation motor for an electric vehicle rather than a conventional non-oriented electrical steel sheet.
A non-oriented electrical steel sheet is disclosed in which Mn and Ni are added to Si to achieve solid solution strengthening in Patent Literature 1. However, it is not possible to obtain sufficient strength even by the non-oriented electrical steel sheet. Further, due to the addition of Mn and Ni, its toughness is likely to be reduced, and sufficient productivity and a sufficient yield cannot be obtained. Further, the prices of alloys to be added are high. In recent years in particular, the price of Ni has suddenly risen due to a worldwide demand balance.
Non-oriented electrical steel sheets are disclosed in which carbonitride is dispersed in a steel to achieve strengthening in Patent Literatures 2 and 3. However, it is not possible to obtain sufficient strength even by the non-oriented electrical steel sheets.
A non-oriented electrical steel sheet is disclosed in which Cu precipitates are used to achieve strengthening in Patent Literature 4. However, it is difficult to obtain sufficient strength. For obtaining sufficient strength, annealing at high temperature is required to be performed in order to once solid-dissolve Cu. However, when the annealing at high temperature is performed, crystal grains coarsen. That is, even though precipitation strengthening by Cu precipitates is obtained, by the coarsening of crystal grains, strength decreases and thus sufficient strength cannot be obtained. Further, due to the synergistic effect of precipitation strengthening and coarsening of crystal grains, fracture elongation significantly decreases.
A non-oriented electrical steel sheet is disclosed in which suppression of the coarsening of crystal grains in Patent Literature 4 is intended in Patent Literature 5. In the technique, C, Nb, Zr, Ti, V, and so are contained. However, at 150° C. to 200° C., being a heat generation temperature range of a motor, carbide precipitates finely and magnetic aging is likely to occur.
A non-oriented electrical steel sheet is disclosed in which by precipitates of Al and N, achievement of making crystal grains fine and precipitation strengthening by Cu is intended in Patent Literature 6. However, Al is contained in large amounts and thus it is difficult to sufficiently suppress the growth of crystal grains. Further, when an N content is increased, a cast defect is likely to occur.
A non-oriented electrical steel sheet containing Cu is disclosed in Patent Literature 7. However, in the technique, a heat treatment for a long period of time, and so on are performed, to thereby make it difficult to obtain good fracture elongation and so on.